Magnetic Determination of Shape Distribution of Single Domain Powders

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Abstract
By demagnetizing a magnetic powder specimen with diminishing alternating fields of various initial amplitudes and by interpreting the results with the Stoner‐Wohlfarth model, one can obtain information about the shape distribution of the particles. We have done this with powders of acicular γ‐ferric oxide at various temperatures; the temperature variation of the results gives information about the size distribution. The sample is first magnetized to saturation; the diminishing ac field is then applied and the remanence measured. Since the particles are not aligned along the applied field, it is necessary to get the equivalent magnetization curve for an aligned sample by operating on the data mathematically. The results indicate that only a small volume fraction of the γ‐ferric oxide particles are small enough to be a paramagnetic. There is a relatively large volume fraction of particles with nearly zero critical field. We believe this is caused by particles which change their magnetization by a mechanism different from the Stoner‐Wohlfarth uniform rotation. On a volume basis, the shape distribution curves peak at a length‐to‐diameter ratio of about 1.6:1, which gives a coercive force of 300 oersteds. The coercive force agrees very well with measured values. The length‐to‐diameter ratio does not agree with electron microscopic observations, which give a mean of about 5:1.

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